Strings & StringBuilder

• String (The Stone Tablet): Imagine carving a sentence into stone. If you want to add a word at the end, you can’t just "insert" it. You have to get a whole new slab of stone and carve the entire new sentence from scratch.

• StringBuilder (The Whiteboard): This is a special tool designed for efficiency. You can write, erase, and append text without needing to buy a new "board" every time. It’s much faster for heavy text manipulation.

Common String Methods

Java gives us a "Swiss Army Knife" of tools to inspect text:

• .length(): How many characters?

• .toUpperCase() / .toLowerCase(): Change the casing.

• .contains("abc"): Does it have these letters?

• .substring(start, end): Cut a piece out of the text.

The Code Example:

public class TextPro {
    public static void main(String[] args) {
        // --- 1. The Immutable String ---
        String greeting = "  Hello Java!  ";
        
        System.out.println("Original: [" + greeting + "]");
        System.out.println("Trimmed: [" + greeting.trim() + "]"); // Removes spaces
        System.out.println("Upper: " + greeting.toUpperCase());
        System.out.println("Starts with 'H'? " + greeting.trim().startsWith("H"));

        // --- 2. The Efficient StringBuilder ---
        // Let's build a long string using a loop
        StringBuilder sb = new StringBuilder();
        sb.append("Alphabet: ");

        for (char c = 'A'; c <= 'E'; c++) {
            sb.append(c).append(" "); // Adding characters efficiently
        }

        // We can also modify the middle
        sb.insert(0, "THE "); // Adds "THE " at the beginning
        sb.reverse();        // Flip the whole thing!

        System.out.println("\nStringBuilder Result:");
        System.out.println(sb.toString());
    }
}

Expected Output:

Original: [  Hello Java!  ]
Trimmed: [Hello Java!]
Upper:   HELLO JAVA!  
Starts with 'H'? true

StringBuilder Result:
 E D C B A :tebahplA EHT

n a real-world app—like a chat app or a search engine—you are processing millions of characters per second. If you use regular String concatenation (str = str + "new text") inside a loop, your app will lag. Using StringBuilder makes your code professional and high-performance.

Key Rule: Use String for simple labels or data that doesn't change. Use StringBuilder when you are building long pieces of text or modifying text inside loops.